Author ORCID Identifier

https://orcid.org/0000-0001-9102-9252

Year of Publication

2019

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Chemical and Materials Engineering

First Advisor

Dr. Yang-Tse Cheng

Second Advisor

Dr. Dibakar Bhattacharyya

Abstract

Rechargeable lithium ion batteries (LIBs) have long been used to power not only portable devices, e.g., mobile phones and laptops, but also large scale systems, e.g., electrical grid and electric vehicles. To meet the ever increasing demand for renewable energy storage, tremendous efforts have been devoted to improving the energy/power density of LIBs. Known for its high theoretical capacity (4200 mAh/g), silicon has been considered as one of the most promising negative electrode materials for high-energy-density LIBs. However, diffusion-induced stresses can cause fracture and, consequently, rapid degradation in the electrochemical performance of Si-based negative electrodes. To mitigate the detrimental effects of the large volume change, several strategies have been proposed. This dissertation focuses on two promising approaches to make high performance and durable Si electrodes for high capacity LIBs.

First, the effect of polymeric binders on the performance of Si-based electrodes is investigated. By studying two types of polymeric binders, polyvinylidene fluoride (PVDF) and sodium alginate (SA) using peel tests, SEM, XPS, and FTIR, I show that the high cohesive strength at the binder-silicon interface is responsible for the superior cell performance of the Si electrodes with SA as a binder. Hydrogen bonds formed between SA and Si is the main reason for the high cohesive strength since neither PVDF nor SA bonds covalently with Si.

Second, the fabrication of high performance Si/polyacrylonitrile (PAN) composite electrode via oxidative pyrolysis is investigated. We show that high performance Si/polyacrylonitrile (PAN) composite negative electrodes can be fabricated by a robust heat treatment in air at a temperature between 250 and 400oC. Using Raman, SEM, XPS, TEM, TGA, and nanoindention, we established that oxidation, dehydration, aromatization, and intermolecular crosslinking take place in PAN during the heat treatment, resulting in a stable cyclized structure which functions as both a binder and a conductive agent in the Si/PAN composite electrodes. With a Si mass loading of 1 mg/cm2, a discharge capacity of ~1600 mAh/g at the 100th cycle is observed in the 400oC treated Si/PAN composite electrode when cycled at a rate of C/3.

These studies on the structure-property-performance relations of Si based negative electrode may benefit the LIB community by providing (1) a guide for the design and optimization of binder materials for Si electrodes and (2) a facile method of synthesizing Si-based composite negative electrodes that can potentially be applied to other Si/polymer systems for further increasing the power/energy density and lower the cost of LIBs for electric vehicle applications and beyond.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2019.414

Funding Information

This research was supported by National Science Foundation (Award number 1355438).

Available for download on Tuesday, May 05, 2020

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